This invention relates to method and apparatus for controlling the configurations of liquid coatings, and particularly for controlling the configuration of a paint stripe covering the side-seam area on the inside of a food can.
In the making of a welded side-seam food container by bending a flat sheet of metal into cylindrical form and welding the resultant side seam, it is usual to coat the side of the sheet which will constitute the inside surface of the can with a food-inert paint or enamel, not only to provide an attractive appearance for the inside of the can but also to render it food-inert so that the can will not react with the contents and thereby produce objectionable or harmful reaction products.
In order to provide bare metal on the portions of the can which are to be welded to each other along the side seam, so as to enable satisfactory welding to take place, the corresponding margins of the original flat sheet are normally left uncoated, and will remain so after the side-seam welding has been completed, thereby forming an uncoated stripe region on the inside of the can adjacent to the linear weld. For the reasons indicated above, it has been found desirable to cover the uncoated side-seam weld area on the interior of the can with an inert coating which will be referred to herein as a paint stripe. It is important that this paint stripe be completely dried and cured before food products are placed into the completed can, so that it will be chemically inert and will contain no volatile components at that time.
It will also be understood that, in the interest of efficient production, it is desirable to move the welded cylindrical cans rapidly, and preferably at a uniform speed, through a paint-stripe applying stage and through subsequent paint drying and curing stages to the mechanism which completes formation of the can, the first element of which is usually a flanger which forms flanges at the opposite ends of the can cylinder; this and subsequent stages in the can formation are not part of the present invention, and need not be described in detail herein.
In order for the paint stripe to provide a good, continuous, adherent coating, it has been found desirable to apply the stripe by means of a paint spray head positioned to spray paint along and over the interior of the side-seam area as the can cylinders proceed along the production line. However, in this event a special problem arises from the fact that the side-seam welder customarily delivers the welded cylinders to its output end with the cylinder axis lying horizontally and with the welded side-seam lying along the top of the horizontally-lying can cylinder. The paint stripe is therefore applied by spraying the paint upwardly inside the cylinder, against the highest or top area of the interior of the cylinder, and both gravity and the pressure of the spray therefore tend to cause the paint to flow downwardly along the curved interior surfaces of the cylinder, away from its desired position, before it has dried sufficiently to set and hold its originally applied position. Further, since the side-seam presented to the sprayer is still hot from its immediately-preceding welding operation, the tendency for the paint to flow downwardly away from the seam area is further exacerbated. This tendency to flow downwardly not only thins the coating in the top region where an impervious continuous coating is desired, but has another important adverse result; at the edges of the stripe, where the downward flow is arrested by the adhesive, cohesive and surface tension properties of the paint and by its cooling, the paint will tend to pile up and form ridges, much like the familiar "sagging" of a wet wood-work enamel. When such a stripe is heated to dry and cure the paint stripe, the thick ridged areas tend to form bubbles, which is highly undesirable from the quality standpoint in the completed product. Such bubble formation is not only undesirable aesthetically, but also may produce little or no coating and reduced adherence of the paint in the areas where bubbles are formed.
The heating for accomplishing the setting, drying and curing of the paint stripe may be performed by high frequency induction heating applied to the upwardly presented surface of the can cylinder, whereby alternating currents in the induction heater elements produce corresponding alternating magnetic fields extending through the adjacent portions of the can cylinder, inducing therein eddy currents which heat the nearby portions of the can cylinder. Among the types of induction heaters which may be used for such purposes are the linear element induction heater and the serpentine element induction heater, described and defined more fully hereinafter.
In general, the linear element induction heater uses conductors which extend in a straight line parallel to the axis of the can cylinder, and may be placed over the side-seam weld area to produce concentrated local heating of this area and of the adjacent paint stripe on the interior of the cylinder. The serpentine element induction heater uses an element which sinuates back and forth across the top of the can, and at its outer reaches may bend progressively farther down around the can circumferentially, so as to maintain its rather close spacing to the outer surface of the can cylinder; such an induction heater element tends to induce more even heating of the can surface underlying the stripe.
It has been found that if the simple linear-element induction heater or the serpentine element induction heater is used according to the prior art as a sole source of heat for the initial heating of the paint stripe, the above-mentioned ridging and bubbling problems are encountered. It has also found desirable to perform the setting, drying and curing steps into three successive stages, namely, first induction heating of the stripe, followed by a "dwelling" or "resting" stage where additional induction heating is not applied and solvents are permitted and encouraged to evaporate quickly, followed by a second induction heating stage in which curing of the paint stripe is completed. While such a three-stage arrangement accomplishes the desired setting, drying and curing operations quickly and effectively, it is found that it does not in itself overcome the above-described problem of paint ridging and bubbling. This drawback of the process has been found to remain whether the first and second induction heatings are conducted with conventional linear induction heating elements or with serpentine induction heating elements.